For what it's worth I remember reading (many years ago) that Anglo-Saxon England was the wealthiest country in the world, on a per capita basis. There were certainly other polities that were richer in an absolute sense - France, the Byzantine Empire, etc - but these all had much larger populations.
It's certainly not improbable when you remember the huge sums of money raised either to fight the Vikings or pay them off. There are also the many hoards of treasure found in England - most notably the Staffordshire Hoard from a few years ago. I'm not aware of a comparable number of hoards being found elsewhere in Europe (albeit this may be pure ignorance on my part!).
As a last point I've also seen it claimed that it was wealth looted from England that paid for the building of the Gothic cathedrals of Northern France, though I'm not sure if this would be easy to prove.